The present invention relates to a device and to a method for reinforcing a pylon foundation against being pulled out, and it is intended more particularly for reinforcing an existing pylon foundation of the so-called “shallow” type.
A shallow foundation is a foundation near the surface that provides the pylon with stability by spreading loads over an area of ground that is large enough. For example, lattice-type pylons generally stand on a foundation made up of four blocks, i.e. four individual blocks of concrete buried, at least in part, in the ground so as to counter toppling moments applied to the pylon in a lever-arm configuration. Changes in regulations concerning the stability of structures are requiring foundations of this type to be reinforced if they are not strong enough.
In general, reinforcement is required only against pulling-out forces. In most cases, the bearing area of shallow foundations is sufficient to withstand compression forces.
Various methods and devices for reinforcing a pylon foundation against being pulled out are already known. Those methods are implemented on existing foundations and seek to compensate for the deficit in pull-out strength of at least one block of the foundation. The term used is “force deficit” which is written Qal below and is expressed in newtons (N).
Several factors can lie behind the deficit Qal, including an increase in the pull-out force to which the foundation is subjected. Such an increase can be due:                to changes in the operating conditions of the foundation (conditions may be climatic, mechanical, geometrical . . . );        to weakening in the characteristics of the ground around the foundation blocks, due to an external phenomenon which may be natural or artificial (storm, earthquake, engineering works . . . ); and        to the difference between the real shape of the foundation and its design shape, due to faulty fabrication of the foundation.        
Depending on the value of the pull-out force deficit Qal that needs to be compensated, recourse is had at present to two known methods.
The first consists in casting a block of concrete around the leg of the pylon or the non-buried portion of the block (if there is one), so as to increase the weight of the foundation by adding the weight of said concrete block. Nevertheless, since it is appropriate to limit the size of the block so as to limit the space it occupies around the base of the pylon, the weight of the block is limited and can serve only to compensate small values of force deficit Qal, generally values less than 20 kilonewtons (kN).
The second known reinforcement method consists in reinforcing the foundation by means of micropiles that are mechanically connected to the legs of pylons and that are thrust deeply into the ground down to a deep substratum presenting good mechanical strength, such as a substratum of rock. That method is described in document FR 2 810 056. The micropiles take on all of the loads applied to the pylons (the existing foundation is hardly stressed any more and is useful only in terms of the weight of its own concrete that it contributes to the assembly). The lateral friction between each micropile and the deep substratum can enable high deficits Qal to be compensated, deficits greater than 1000 kN. However, the size of the micropiles, their technical nature, and the means needed for putting them in place make this second method very expensive. In practice, pylons are generally never located close to roadways and it is often necessary to use heavy equipment on agricultural or sloping ground.